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  1. #1
    Join Date
    Feb 2006
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    Lightbulb Quick notes on the STF lens

    There may come a day when you are looking in the lens store, intrigued with the "STF" 135 lens. Hey, it could happen. There may even be a chance you know someone that has one, already. Well, familiarization is halfway to real appreciation, in my estimation, so here are a few casual notes regarding the operation of this gem.

    How to operate the STF lens.

    First off, I would like to say that this is a very misunderstood lens. I kind of laughed as I bought it, when I was told by the “seller” that he had no real idea how it worked, did not have time to learn and would be a lot happier just using the SONY CZ 135mm f/1.8.

    Well, I own a SONY CZ 135mm f/1.8 and I have to say … it may be same working focal length, but they are completely different animals.

    Unlike the Zeiss, inside the heart of the “STF” lens is an Apodization element and two aperture blade sets. The Apodization element is a fixed portion of the lens, about halfway through the light path, and basically creates the intense smoothing of the bokeh. Of the two aperture rings, one is controlled by the camera, when you select your aperture by the camera. The other set of blades is controlled by the "STF" ring, which runs around the circumference on the rear of the lens.

    With this “STF” ring in the “A” position, which is marked in green on the lens’ rear aperture control ring … the camera’s aperture setting has control of the rear set of blades. If you select the STF settings, the rear set of camera controlled aperture blades automatically retract to their fully open position and the front set of aperture blades take over, for a much more controlled (“stepless”) aperture value of f/4.5-6.7

    What I mean by a “much more controlled” value is that where the normal camera settings jump (or “step”) in one-third f/stop increments, the STF ring smoothly moves throughout the range of 4.5-6.7 (“stepless”) without being locked to these one-third f/stop settings. So if you want ~f/4.7 aperture, you can simply turn the manual aperture ring and get there. Say you want a little tighter aperture, say ~f/4.9 … simply turn the STF control ring a little more and get it. There are “detent” points (@ f/4.5, f/5.6 and a stop at f/6.7) as you turn the ring, so you can kind of “know” how far you are with the aperture without having to look.

    The internal camera’s indications (EXIF data) that your will see on the LCD or in the viewfinder, will be @ f/4.5, 5.6, 6.3 as you go, as the camera is not calibrated to completely follow the actual STF value through its range. Switching the STF control ring back to the “A” position, you can get the camera’s normal aperture settings of f/4.5, f/5, f/5.6, f/6.3, f/7.1 … on up to f/32

    About f/2.8 aperture … forget it. With the addition of the twin aperture rings inside this lens, the light is cut down to f/4.5 wide open. If you want f/2.8 or wider … look at the Zeiss.

    Again, the manual control of the Smooth Transition Focus only takes place in the range f/4.5-6.7. If you want or need a tighter aperture for your image, the Apodization element (see detail below), within the lens, works with standard STEPS with the camera’s aperture. While the effective DOF is broadened the smooth bokeh performance of the lens still offers an excellent gradual diffusing effect without losing its shape.

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    Well, if you have taken the time to read this, I hope it has helped in your appreciation of what this lens can do over the others. It is a manual focus lens that requires the photographer to "conceive" his shot, other than just record it. If you are in a hurry, it may be a waste of your time. One of the unique aspects of this particular "manual" lens is that it reports back the aperture information to the camera's metering system (but NOT the ADI information of an eight contact or "D" lens, therefore a TTL-flash solution will be inaccurate, if selected, as it could go for the extreme distance of 26'. Personally, I recommend calculating a "manual" flash exposure.). The fact is, most other modern manual lenses do not provide this aperture data (as they were never designed to and lack the electrical extras - connections and whatnot - to make that happen) and require FULL manual solutions (aperture, shutter speed & ISO) or a crippled A-mode (if available) to operate with some automated-metering capability. Since the Full Frames (α850/α900) do not have a crippled A-mode, this lens still provides them the necessary aperture data for all metering modes (AUTO, P, A, S, M) to operate as designed.
    Last edited by DonSchap; 12-28-2009 at 12:57 PM.
    Don Schap - BFA, Digital Photography
    A Photographer Is Forever
    Look, I did not create the optical laws of the Universe ... I simply learned to deal with them.
    Remember: It is usually the GLASS, not the camera (except for moving to Full Frame), that gives you the most improvement in your photography.

    flickr & Sdi

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